Mechanical analogies to electronics symbols are common in other engineering disciplines. We might refer to this one, then, as akin to a battery with an internal short circuit?
I’ll warn you upfront that this particular blog post has nothing specific to do with electronics (aside, I suppose, from the potential for electrocution caused by a water-soaked calamity). That said, I’ll also postulate upfront that (IMHO, at least) it has a great deal to do with engineering in general, specifically as it exemplifies the edge and corner cases that were the subject of a 2.5+ year back previous post from yours truly. Read on or not, as you wish. That said, I hope you’ll proceed!
I kicked off that prior writeup with the following prose:
Whether or not (and if so, how) to account for rarely encountered implementation variables and combinations in hardware and/or software development projects is a key (albeit often minimized, if not completely overlooked) “bread and butter” aspect of the engineering skill set… I’ve always found case studies about such anomalies and errors fascinating, no matter that I’ve also found them maddening when I’m personally immersed in them!
Speaking of the personal angle…and immersion, for that matter…😂
At our peak, my wife and I have had (several times so far…blame me, not her) up to five four-legged mammal companions concurrently sharing our residence with us. Therein explaining the sizeable (4-gallon/15-liter reservoir) Petmate Aspen Pet Lebistro Cat and Dog Water Dispenser that we bought through Amazon at the beginning of 2020:
Amazon’s packaging robustness can be hit-and-miss; when this particular order arrived at our front door, the reservoir and base were detached and loose. And the outer box contained no packing material, far from inner boxes for either/both constituent piece(s). Unsurprisingly, therefore, the reservoir tank had a dent in one corner (the below is a more recent picture…keep reading):
I pushed it back into place as best I could:
and then filled-and-tested the tank, which still seemed to be watertight. And then, driven by a broader longstanding abhorrence for sending functionally sound albeit cosmetically compromised stuff to the landfill, I decided to keep it and press it into service, accompanied by a successful partial-refund request made to Amazon customer service.
Fast-forward six years. We’re down (for the moment, at least) to only one (canine) companion, a factoid which as you’ll soon see likely ended up being key. And we started finding puddles of standing water in proximity to the water dispenser on the (watertight vinyl, thankfully) laundry room floor. Did we initially accuse the dog of bumping into the dispenser, causing spills? Yes, we did. Did subsequent observation convince us that our initial theory was off base? Yes, it did. And did we then feel badly for unjustly initially blaming the dog? Yes…we did. Bad humans. Bad!
In-depth painstaking engineering analysis (cough) eventually led to the realization that the water spills were preceded by slow-but-sure filling of the bowl all the way to the lip (and then beyond, therefore the puddles), versus the inch-below-the-lip level that the dispenser traditionally stuck to. But what had changed? Figuring this out required that I first learn about how gravity water bowls function in the first place. How do they initially fill only to the inch-below-the-lip level, and how do they then automatically maintain this level as the water is consumed by canine and feline companions, until drained (if one of the humans had forgotten to refill it, that is)?
I learned the answer from, as I’m more generally finding of late, Reddit. Specifically, from a post in the cleverly named “Explain Like I’m Five” subreddit (I’m doing my best not to take offense) titled “How do self-filling/gravity fed pet water bowls not overflow and spill everywhere?”. The entire discussion thread is fascinating, again IMHO, containing exchanges such as the following:
- ender42y: This works for a stack up to 32 ft or 9 meters tall (at standard atmospheric pressure) at which point the top of the water tank would actually start to form a vacuum.
- bloc97: It is a bit shorter in practice as the water will start to boil at ~2 kpa (assuming 20c).
- MindStalker: That’s exactly why you are limited to a column that’s about 9 meters tall, anything above that boils away.
- bloc97: Yes, as there are two processes that determines the column height (density and vapor pressure of the fluid), we just need to make sure not to confuse the two.
Again: 😂. That said, have I yet admitted what a devoted follower of the TV personality Mr. Wizard I was as a wee lad (we didn’t have YouTube back then)?
That admission explains (more than) a few things, yes? Speaking of vacuums, here’s the “money quote” from that Reddit post thread, with kudos to Redditor nestcto:
Recapping the basics, the opening acts as both the exit for the water, and the entrance for air. The air is obviously needed because under normal circumstances, you can’t just have nothing in the bottle. The water must be replaced with something. That’s where the air comes in.
So making the water leave the bottle is easy. You just have to make sure the water is creating more outward pressure to leave the bottle, than the vacuum inside trying to replace it with air.
To keep the water in, you have to make sure the water can’t create more pressure to leave the bottle than the vacuum trying to suck in air. This is more difficult because water is heavy, so gravity pushes it down a lot. The more water, the more pressure. The more pressure, the easier to overcome the vacuum.
Viscosity is a factor here as well, that I won’t go into too much. Basically, the thicker something is, the harder it is to get through a small opening.
Water isn’t very thick, but it’s much thicker than air. So there’s a point where the opening is small enough that water has trouble getting through it without some pressure behind it. The force of gravity isn’t strong enough to push the water through the small opening, and the internal vacuum is too weak to suck air in since no water has left yet to create a vacuum. So there’s a standstill.
When this happens, you may notice that you can actually make the water flow outwards by agitating the bottle. Take a needle or toothpick, and swish it around the opening. You’ll notice that some water leaves the bottle. This causes a small vacuum to replace the water. Which sucks in air. The air displacing the water to rise upwards can destabilize gravitational pressure towards the opening, causing more water to leave, and more air to come in, and next thing you know, the whole thing is emptying due to a cycle of pressure. Water out. Vacuum created. Air in. Vacuum satisfied. Water out. Vacuum created…and so on.
Now, the point at which you reach that standstill depends on a LOT of factors. But it’s pretty much always a lot easier to accomplish with less water, because you have less downward pressure to fight against due to gravity.
Ok, that’s all well and good. But it still doesn’t explain why my “self-filling/gravity fed pet water bowl” “destabilized”, as nestcto referred to it. For that, keep scrolling through the thread (admittedly, I particularly resonated, for different reasons, with the last two):
- MrBulletPoints: Yeah if you were to jam something sharp into the top of that setup and make a hole, it would allow air in and the bowl would definitely overflow
- verronbc: Yeah… I like to call myself smart sometimes. The first bowl like this we had our dumb 70 lb dog was scared of the bubbles it made after he drank a bit out of the bowl. My simple solution, “Oh, I know I’ll just drill a hole in the top then the air will fill from the top” yeah in a moment of weakness I forgot exactly why and how these work and caused a lot of water to drain on the floor. My girlfriend still teases me about it.
- [deleted]: That’s why you have to punch a hole in the beer can before you shotgun it. It’s the same concept.
- stoic_amoeba: As an engineer, I’ve genuinely had this idea cross my mind because the bowl is HEAVY when you fill it all the way. Also, as engineer, I’m a bit ashamed I didn’t immediately realize how bad an idea that’d be. I haven’t done it, thank goodness, but I should know better.
Ahem. At least some of you likely realize what happened next. I went back and looked at that reservoir again, eventually realizing that after six years, the dent had finally become slightly compromised. If I filled the reservoir, turned upside-down from its normal position (so it was resting on its top), left it in the sink and waited a long time, I’d eventually find that a few drops of water had leaked out of it.
The same compromise was true (in reverse) for air, of course, when it was oriented correctly and in place. And it very well might have been like this for a while, counterbalanced by the frequent water intake of multiple pets. Drop the count down to one dog, though, and…puddles. We replaced it with a smaller dispenser from the same manufacturer, the first example of which also arrived from Amazon dented, believe it or not (I shipped it back for replacement this time):
and we’re happily back to an always-dry floor again.
I’ll close with a few photos of the original base, both initially intact and then disassembled:
The way these things work is that, after filling the reservoir with water, you screw on the lid:
then turn it upside down and quickly rotate it to lock it in place here:
The lid’s hole diameter is also key, by the way, as I learned one time when I put the reservoir in place without remembering to screw on the lid first (speaking of water all over the floor)…
Note the gap around the repository where the lid fits. I’m guessing this is where the air comes from to replace the displaced water in the reservoir, but I haven’t come across another Reddit thread to supplement my ignorance on this nuance. Reader insights on this, or anything else which my case study has stimulated, are as-always welcome in the comments!
—Brian Dipert is the Principal at Sierra Media and a former technical editor at EDN Magazine, where he still regularly contributes as a freelancer.
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